Data storage media are commonly used for storage and retrieval of data, and come in many forms, such as magnetic tape, magnetic disks, optical tape, optical disks, holographic disks, cards or tape, and the like. Magnetic tape media remains an economical medium for storing large amounts of data. In magnetic tape, data is typically stored as magnetic signals that are magnetically recorded on the medium surface. The data stored on the magnetic tape is often organized along “data tracks,” and read/write heads are positioned relative to the data tracks to write data to the tracks or read data from the tracks.
Servo patterns refer to signals or other recorded marks on the medium that are used for tracking purposes. In magnetic tape, the servo patterns are recorded in specialized tracks on the medium, called “servo tracks” to provide reference points relative to the data tracks. A servo controller interprets detected servo patterns and generates position error signals. The position error signals are used to adjust the lateral distance of the read/write head relative to the data tracks so that the read/write head is properly positioned along the data tracks for effective reading and/or writing of the data to the data tracks.
A servo writer writes the servo patterns and may include a magnetic servo recording head and various transport guides to precisely guide the medium over the head. If the medium movement relative to the servo recording head is not well controlled, then the recorded servo pattern may not provide reliable positioning information during use of the medium. For these reasons, servo writers typically use verification heads and electronics to insure that the servo pattern is accurately recorded. In particular, a servo verify head is used to read back the servo markings to detect flaws in the written servo signal.
Time-based servo techniques refer to servo techniques that make use of time variables. Time-based servo techniques are particularly effective for magnetic tape, which typically feeds past transducer heads at a constant velocity. For example, N-shaped servo markings, servo markings such as “<<<>>>” or “////\\\\,” or the like, have been developed for time-based servo techniques. Such markings are typically formed in a servo track of the magnetic tape.
When time-based servo techniques are used, the time offset between detection of two or more servo marks can be translated into a position error signal, which defines a lateral distance of the transducer head relative to a data track. The time between servo marks depends on the distance between the recorded servo marks and the velocity of the magnetic tape. In order to accurately measure the tracking error recorded into the servo pattern, the servo writer must have accurate tape velocity control, an accurate signal peak detector, and an accurate clock with which to measure time.
Traditional analog peak detection can provide accurate peak detection at low frequencies, but may suffer from inaccuracies, particularly as the signal frequency increases. Digital circuitry has been developed to measure time between signal peaks corresponding to the servo signals. This digital circuitry typically needs to operate at very high clock rates in order to achieve accurate readout of the servo signals. Some mixed analog and digital electronic components that can provide good readout capabilities also exist. However, new techniques for reading servo signals are highly desirable, particularly for next-generation magnetic media.